Oxidizing catalysts



Patented Feb. 18, 1936 UNITED I STAT-ES PATENT oerics 2,031,: oxImzrNG oA'rA rsTs No Drawing. AppIication'June ZS, 1931, Serlal'No. 546,928

I and especially to such catalysts for use at elevated temperatures. ,While the inventionis unlimited to any particular use, it has been found to be especially advantageous in the treatment of exhaust gases from internal combustion engines for eflecting combustion of their combustible constituents, including carbon monoxide and other toxic gases, and is accordingly so described herein.

Catalysts used for the oxidation of carbon monoxide contained in the exhaust gases from internal combustion engines are subjected to extremely severe conditions of use. In addition of of treating these gases to eliminate their combustible constituents by subjecting them, admixed with oxygen, e. g. air, to the action of an artificially prepared chromite, and specifically iron chromite. Actual tests with antombiles have shown that the iron chromites disclosed therein operate satisfactorily in the manner described in the patent. It has been found, however, that after extended use their starting temperature is about 100 0., and therefore means other than the heat of the raw gases is needed to bring them to starting temperature. Hence, while they fully meet the, main requirements of a catalyst for this use, and are accordingly fully operative to containing relatively large amounts of this and capable of satisfactory use, they lack the toxic constituent these gases also contain large ancillary desideratum of continuously low startamounts of other combustibles, such as unburned ing temperature. gasoline and oil, and products resulting from It is among the objects of the present inven-' their decomposition or partial combustion. They tion to provide catalysts which are capable of also contain'water vapor. Experience has shown effecting oxidation reactions, are adapted for go that it is not possible to selectively oxidize carsustained use at elevated temperatures and parbon monoxide in such gas mixtures, and that all ticularly for eifecting complete oxidation of the of the combustibles are bur ed in completely concombustible constituents of internal combustion verting the carbon monoxide to dioxide. In conengine exhaust gases, are inexpensive and easily 23 sequence very large amounts of heat are liberprepared, highly active, capable of functioning as ated in the catalytic combustion of .the carbon in the presence of water vapor, remain efficient monoxide in internal combustion engine-exhaust over long periods of time when used either congases. These gases also contain water vapor, and tinuously or intermittently, and which possess low this together with oil and the like, generally and substantially constant starting temperatures.

33 tends to reduce the efliciency of catalysts. There- A particular object is to provide a method of fore, to be effective for this purpose, the catamaking catalysts of the type just referred to lysts must be capable of continuous high activity which is simple, readily performed, and is adaptat elevated temperatures, and in the presence of ed to provide supported and non-supported catathe materials referred to. lysts.

A further requirement of such catalysts is that The invention is predicated p y discovery 35 they shall be capable of continuous and complete that oxygen compounds of copper and chromium oxidation of the combustibles in these gases after in propo ons Corresponding Substantially 150 being repeatedly cooled from and re-heated to copper chromites form highly active oxidizing such temperatures. That is, internal combustion catalysts embodying the stated objects, and furengines usually are not-operated continuously. ther upon my discovery that such copper chrolo For this reason the catalysts must be capable of mite catalysts may be prepared simp y d readefiicient operation when used intermittently, i. e., ily by reduction of copper chromates. The cataeach time the en ine is run, whether such interiysts contemplated by the invention possess low mittent operatio be of but a few minutes or of starting temperatures, which are not apparently many hours duration. increased through extended use, and which are maddition to the foregoing requirements, praclow enough to adapt them to be heated suflitical considerations render it desirable also that ciently by internal combustion engine exhaust these catalysts possess a relatively low starting gases to cause them to function, without external temperature, in order to avoid the use of'acces-- heating. This characteristic, combined with sory catalyst heating devices-whichwould othertheir other properties, renders them especially so wise be necessary. Ideally the catalysts should suited for elimination of combustible constituents be capable of initiating and continuing combusfrom exhaust gases of internal combustion ention of thecombustible constituents in these gases gines. v upon being heated only by thegases supplied to Various copper chromites are embodied in the d them. For the same reason the starting teminvention, and they may be made in various 55 perature should remain constant, or substantiall ways. For instance, a suitable catalyst may be so, after long continued use, whether continuous prepared from a reaction product of copper hyor intermittent. droxide and chromic acid in' proportions cor- In my U. S. Patent No. 1,789,812, issued Januresponding to copper chromate. I have found that copper hydroxide dissolves readily in 60 my 20, 1930, Ihave disclosed and claimed a methaqueous solutions of chromic acid to form a solution of an oxygen compound of copper and chromium which upon being evaporated and heated is apparently converted to a catalytically active copper chromite. Apparently an acid copper chromate is formed, and this is then reduced to chromite, although I do not limit myself by this supposition. Copper carbonatemay be used also, instead of the hydroxide.

The copper hydroxide or carbonate dissolves particularly readily if in a freshly precipitated condition. In the preferred embodiment the cupric compound is used, the chromite being formed by evaporation of the solution to dryness, and heating the residue, preferably by ignition at about 1000 to 1100 C. The dried. residue is then broken up to provide granules of the copper chromitewhich may be used directly for catalytic purposes.

If preferred, supported catalysts may be prepared by immersing a suitable support in the chromate solution, and drying and heating the wetted support. A particularly suitable form of supported catalyst may thus be prepared by immersing a metallic screen in the solution, drainsuch chromites, there may be described the prep- I aration of what appears to-be cupric meta chromite. To this end, one moi of cupric hydroxide (or carbonate) is reacted with two mois of chromium trioxide in aqueous solution. For example, 296 grams of hydrated copper nitrate [Cu(NO;)a.6HaO] dissolved in a large excess of water is precipitated with half-strength ammonia, and to the resultant suspension of cupric hydroxide there is added suificient chromic anhydride preferably in concentrated aqueous solution, to form cupric Preferably the acid chromate, Cu(HCrO4)s. is formed, and accordingly a solution of 200 grams of chromic anydr'ide (010:) in suiiicient water to dissolve.

it isadded to the copper hydroxide suspension. Upon vigorous stirring the copper hydroxide dissolves rapidly, forming a deep red solution, which is evaporated to dryness and heated, as described hereinabove.

Actual tests of copper chromite prepared in this manner has shown itto be a good oxidation catalyst, and to properties which make it a desirable catalyst for the treatment of internal combustion engine exhaust gases. Thussuchgasespassedthroughabedofgranules of the catalyst bring it up to starting temperature, and in the presence of oxygen it then begins to function at high eiiiciency. The gases being mixed with air in an amount at least corresponding to that theoretically necessary, the

catalyst causes continuous complete combustion of all of the combustible constituents of the gases.

'Whether the engine be operated continuously procedure. I now believe that this precipitated chromate forms a basic chromite upon being heated.

In actual tests of these copper chromite catalysts all of the gases from a standard internal combustion engine were passed through a pipe about '4; inch in diameter containing a 3 inch length of copper chromite granules graded to about 6 to 10 mesh. The gases were mixed with about percent of air before being passed over the catalyst, and temperature and carbon monoxide data were recorded automatically. The

tests with the meta chromite made by the solution procedure showed that as soon as the catalyst reached about 280 C. the temperatm'e in the catlyst bed, as measured by thermocouples, increased abruptly, showing that it had commenced to eifect complete oxidation. Simulta- This bustion of all of the combustible constituents in the gases began at approximately 280' 0., as

shown by the automatic recorder. Inasimilarseriesofteststhebasicchromite Three hundred hours total use of this h monoxide content of the cataiystdidnotdestroythisabiiitytostartat thislcwtenrperatin'e,andthentofunctionper- 'liresea'ndothercopperchr omitecatalystsem bodied in the invention are capable not only of satisfactorily oxidizing all of the combustible and toxic constituents from internal combustion engine gases, during both continuous and interapparentiy do not have any deleterious eifect. For example, the chromites made by the solution procedure have been heated to 1150' 0. without affecting their emciency. Most catalysts are rendered useless for intermittent use after being subjected to such temperatures. I

The catalysts described'herein have been referred to as chromites, and the'substances from whichthey are preferably formed as chromates, this terminology having been adopted for brevity ofreference, and because ofthemannerinwhich the catalysts are It appears probable, forinstancethatcopperchromateisintact formed whcnths copper hydroxide dholvea or mittent operation thereof, but high temperatures reacts with the chromic acid, and that upon" being heated this is reduced to a chromite. Thus in the solution procedure described the cupric acid chromate probably breaks down to form a chromite which corresponds to' the empirical formula Cu0.CrzO:' or Cu(CrO2)z, while that referred to as basic chromite is probably (C110) act-203.

Furthermore, my tests have shown that when the proportions of materials used are varied substantially from those adapted to form a chromate,

' and ultimately a chromite, the resultant material is not as good a catalyst as when the product conforms substantially to chromite proportions. Moreover, tests of mechanical mixtures of the oxides constituting the supposed chromite in proportions corresponding thereto have shown them to be inferior and incapable of the results obtained by the catalysts prepared in a manner adapted to produce a'chromit'e. In view of all of these facts, it appears probable that the catalysts are actually chemical combinations of oxygen compounds of copper and chromium, and probably chromites. It will be understood, however, that they may be other than the compounds named, and that the materials from which they are formed may not be chromates, but that these terms refer to the products of the type and embodying the characteristics described, whatever from copper compounds other than the hy-- be their actual chemical nature.

As has been noted, the chrom'ites may be made droxide. Therefore the term copper hydroxide used in the claims is to be understood as comprehending such compounds, that term, being used for conciseness of reference, and not as restrictive .to the hydroxide.

It has been found also thatthe copper chromites may be caused to start functioning more rapidly if they are preliminarily treated with a reducing atmosphere, e. g. the raw exhaust gases,

unmixed with oxygen, or containing oxygen in.

an amount insnfllcient to effect complete combustion. The ,most suitable means of doing this is to mix the gases flowing to the catalyst'with 1 an insufiicient amount of air for complete combustion, the balance of the air needed being in- -troduced farther along the catalyst bed. The

eifect of this reducing treatment, as shown by tests which have been made, is to lower, or depress, the starting temperature, thus causing the catalyst to eifect oxidation more rapidly thanwhen not so'treated. The tests showed that the starting temperature 0! the meta chromite is thus lowered to about 225 C.,-andthat of the basic chromite'to about 150 0. While this modification is not necessary for most purposes, the catalysts functioning satisiactorllyin the manner described hereinabove, it may usedefor special circumstances.

. The ease of production,cheapness, ability to remainactive at high temperatures for long periods of time, and: function emciently alter repeated heating and cooling, adapt the catalysts ,provided by the invention particularly for use inthe treatment ofinternai engine exhaustgamaswillbeevldentfromwhat'has been said hereinbefore. However, they are qual- "1y applicable for other purposes'where anoxidatloncatalystis-needed, forexampleintheconversionofsnli'urdioxide tosulfurtrloxide; likewise; the copper chromites provided by this invention have been found to be especially valuable catalysts for surface combustion processes.

Other applications will suggest themselves to I those skilled in the art.

This application is a continuation in part of my copendingapplicaticn Serial No. 538,003, filed May 16-, 1931.

According to the provisions of the patent statute's, I have explained the principle of my invention, and have described what I now consider to represent its best embodiment. However, I desire to have it understood that, within the scope ofthe appended claims, the invention may be practiced otherwise than as specifically described.

7 I claim:

' 1. A method of continuously and completely oxidizing carbon monoxide comprising passing a streamof carbon monoxide and oxygen into con- I tact witha catalyst comprising artificially prepared copper chromite.

2. A method of continuously and completely oxidizing carbon monoxide comprising passing a stream of carbon monoxide and oxygen into con-v tact with a catalyst comprising artificially prepared -cupric chromite heated to a temperature above about 250 Cu 7 3. A method of continuously and completely oxidizing carbon, monoxide comprising passing a stream oi carbon monoxide and oxygen into contact with a catalyst comprising artificially pre- .tact with a catalyst comprising copper chromite prepared by dissolving a. soluble compound of copper in chromic acid solution in proportions adapted to form copper chromate, and heating the product to form copper chromite catalyst,

the catalyst being heated to a temperature above about 250 C.

formula 6. A method of continuously and completely I oxidizing carbon monoxide comprising passing a stream of carbon monoxide and oxygen into contact with a catalyst comprising an oxygen compound of copper and chromium prepared by reacting cupric hydroxide with chromic acid solution to procipitate cupric chromate and heating said chromate to convert it to chromite catalyst, the catalyst being heated to above about 250 C.

'I.'A method of continuously and completely oxidizing the combustible constituents of internal combustion engine exhaust gases during intermittent operation of the engine. comprising the gases in a substantially uncooled condition together with air suiilcient for said oxidatiom'into contact with the catalyst comprising artificially prepared copper chromite.

- Josmammazm. 

